The present invention relates generally to memory arrays used in integrated circuit devices, and specifically to built-in mechanisms for replacing defective cells in such a memory array.
The use of embedded memory arrays in Very Large Scale Integrated (VLSI) circuit devices is increasing. Such memory arrays enhance the ability of devices to support high-capacity and high-bandwidth applications. Memories, however, are typically subject to higher process defect-related failures than other elements of VLSI design, such as logic gates. Therefore, the use of embedded memory arrays typically makes devices more susceptible to failures due to manufacturing defects. As total memory on a device gets larger, the memory yield, and hence the device yield, may be dramatically reduced.
It is common practice to include redundant rows and columns in an embedded memory array. These redundant elements can be activated to repair defects inside the memory array. Typically, as part of the device manufacturing flow, the memories are tested, and defective bits in the arrays are detected. The memory array is then repaired by replacing the defective bits with rows or columns selected from the redundant rows and columns. The selection information (i.e., which rows/columns to use) is stored in a special non-volatile memory, typically implemented as a set of dedicated metal fuses, which are fabricated as part of the device. Programming the fuses requires a special step in the manufacturing process, wherein specialized and costly laser equipment is used to blow out selected fuses. Although fuse technology can help to increase device yield, the fuses themselves occupy a substantial area on the chip, therefore increasing device size. The additional manufacturing steps involved in blowing the selected fuses increase cost and can themselves reduce yield.
As an alternative to fuse technology, some VLSI devices with embedded memory arrays include Built-In Self-Test (BIST) circuits, which work together with on-chip repair logic to bypass faulty cells in the array. Such schemes are referred to generally as self-repair systems. As in fuse-based repair schemes, the memory array used for self-repair includes redundant rows and, in some cases, redundant columns. When the BIST circuit detects a fault at a given address in the array, it directs the repair logic to substitute one of the redundant array elements (typically either a cell or an entire row) for the faulty element. Thereafter, reads and writes addressed to the faulty array element are redirected by the repair logic to the redundant element that has been substituted for it.
A variety of self-repair schemes are described in the patent literature. For example, U.S. Pat. No. 5,764,878, whose disclosure is incorporated herein by reference, describes a built-in self-repair system for embedded memories, which is triggered automatically on power-up of the computer in which it is installed. The system includes a BIST circuit that tests for defective row memory lines or defective I/O memory blocks. (An I/O memory block is described as comprising at least one column memory line, but typically includes a plurality of column memory lines grouped together to form an I/O memory block.) When the address of a defective row line or I/O block is detected, it is dynamically repaired by a fault-latching and repair execution circuit, which reroutes the address locations of the defective row or block to new address locations associated with redundant rows or blocks.
The need to look up addresses of redundant rows that are used to substitute for defective rows can substantially increase the access time to the memory. U.S. Pat. No. 5,920,515, whose disclosure is incorporated herein by reference, describes a register-based redundancy circuit and method for built-in self-repair in a semiconductor memory device, which is aimed at solving this problem. The redundancy circuits are associated with failed row address stores to drive redundant row word lines, so that it is not necessary to supply and decode a substitute address in the normal way. The patent states that by providing redundancy handling at the predecode circuit level, rather than at a preliminary address substitution stage, access times to the memory array are improved.
As another example, U.S. Pat. No. 6,011,734, whose disclosure is incorporated herein by reference, describes a fuseless memory repair system, which uses a BIST circuit to determine a specific address at which a memory failure is detected. The address is stored in a latch. During normal operation, the address stored in the latch is compared to addresses being accessed in the memory. When a match occurs, a xe2x80x9cHITxe2x80x9d signal is generated, which disables selection of the defective row in the memory array. A redundant row select signal selects the redundant row to replace the defective row.
U.S. Pat. No. 6,259,637, whose disclosure is also incorporated herein by reference, describes a method and apparatus for built-in self-repair of memory storage arrays. The memory array is produced with a number of redundant rows and redundant columns. A test circuit coupled to the memory array tests the memory cells in the rows of the array. In case of test failure, an error detection unit provides an error indication identifying the bit that failed. A control block keeps track of the errors, and decides whether to repair the failure with one of the redundant rows or the redundant columns.
U.S. Pat. No. 6,304,499, whose disclosure is likewise incorporated herein by reference, describes a semiconductor memory having redundant units of memory cells, along with a method of self-repair. A self-test unit carries out a functional test of the memory cells with a defined memory-retention time for the memory cell contents. The results of the test are analyzed to determine which of the normal memory units are to be replaced by redundant units. The memory units are programmed in accordance with the analysis result.
All the self-repair schemes described above require complex BIST circuits to identify the addresses of the faulty cells, along with address translation logic to redirect read and write data from faulty cells to the redundant cells that are substituted for them. Such BIST and addressing circuits occupy a substantial area on the chip, thus increasing the overall device cost and potentially reducing yield. Furthermore, as noted in the above-mentioned U.S. Pat. No. 5,920,515, the logic needed to identify reads and writes addressed to faulty cells and to look up the substitute redundant cell addresses reduces the access speed of the memory array. There is a need for a self-test and repair mechanism that overcomes these limitations of the prior art.
It is an object of the present invention to provide improved methods and device architectures for self-repair of memory arrays.
It is a further object of some aspects of the present invention to provide built-in self-repair circuitry for an embedded memory array that requires a minimal amount of space on the chip on which it is fabricated.
It is yet a further object of some aspects of the present invention to provide built-in self-repair circuitry for a memory array that does not significantly affect the access time of the array.
In preferred embodiments of the present invention, an embedded memory unit comprises an array of memory lines (rows and columns), including a redundant memory line, preferably a redundant column. In other words, assuming the memory unit is designed to hold n lines of data, the array actually comprises n+1 lines of memory cells. Selection circuitry is coupled to the array so as to select n of the n+1 lines to which the data are to be written and from which the data are to be read.
To determine which lines to use, a Built-In Self-Test (BIST) circuit writes test data to the array and then reads the data from the array. The BIST circuit compares the data read from the array to the data written thereto, and generates a binary (pass/fail) output indicating whether or not the data read from the array corresponded correctly to the test data written to the array. When the BIST output indicates that the self-test failed, a repair machine drives the selection circuitry to select different sets of n of the lines to hold the data. For each new selection, the repair machine triggers the BIST circuit to retest the memory array, until the binary output of the BIST circuit indicates that the data read from the array correspond correctly to the test data written to the array. In this case, the repair machine concludes that the faulty line has been repaired, and this selection is maintained during normal operation of the memory unit. If the selection circuitry cycles through all of the lines without passing the self-test, the repair machine returns a failure signal, indicating that the fault is irreparable.
Preferred embodiments of the present invention thus provide a self-repair mechanism for embedded memory arrays that is simple, reliable and requires little xe2x80x9creal estatexe2x80x9d on chip. Unlike self-repair circuits known in the art, which use complex BIST circuits to identify the addresses of faulty cells, the present invention requires only a minimal BIST circuit with a binary output. The BIST output enables the repair machine to know whether a fault exists and, if so, to determine when the fault has been successfully repaired. The selection circuitry adds little or no latency to memory readout, particularly when it is used to select a redundant column, rather than using redundant rows as in most self-repair mechanisms known in the art.
There is therefore provided, in accordance with a preferred embodiment of the present invention, an embedded memory unit, including:
a memory array having a design size given by an integer n, the array including:
a matrix of memory cells arranged in n+1 lines; and
selection circuitry, coupled to select n of the n+1 lines to which data are to be written and from which the data are to be read;
a built-in self-test (BIST) circuit, coupled to test the memory array by writing test data to the memory array, reading the data from the memory array, and comparing the data read from the array to the test data written thereto, and adapted to generate a binary output indicating whether or not the data read from the array correspond correctly to the test data written to the array; and
a repair machine, coupled to receive the binary output of the BIST circuit and to drive the selection circuitry to select different sets of n of the lines while driving the BIST circuit to test the memory array, so as to find one of the different sets for which the binary output indicates that the data read from the array correspond correctly to the test data written to the array.
Preferably, the memory array is designed to store words of n bits in respective rows of the matrix, and the n+1 lines include n+1 columns of the matrix. Further preferably, the repair machine is adapted to find one of the columns that contains a fault, responsive to the binary output of the BIST circuit, and to drive the selection circuitry to select the n of the columns so as to bypass the column containing the fault. Most preferably, the repair machine is coupled to output a selection value indicating the columns to be selected by the selection circuitry, and the selection circuitry includes a first plurality of selectors, each coupled to a respective one of the columns of the matrix, and further coupled to receive two of the bits in each of the words to be written to one of the rows, and to select one of the two bits to write to the respective one of the columns responsive to the selection value. Additionally, the selection circuitry preferably includes a second plurality of selectors, each coupled to receive the bits read out from two of the columns of the matrix, and responsive to the selection value, to select one of the bits to include in a word that is output from the array.
Preferably, the repair machine is adapted to find one of the lines that contains a fault, responsive to the binary output of the BIST circuit, and to drive the selection circuitry to select the n of the lines so as to bypass the line containing the fault. Further preferably, the repair machine includes a repair register, which is adapted to hold a selection value indicating the line containing the fault, and the selection circuitry is adapted to select the n of the lines responsive to the selection value. Most preferably, the repair machine is adapted to vary the selection value so as to cause the selection circuitry to bypass each of the lines of the array in turn, and to drive the BIST circuit to test the memory array for each of the lines that is bypassed. Additionally, the repair machine is preferably adapted to vary the selection value and to drive the BIST circuit so as to test substantially all of the different sets of n of the lines in the array, and to determine that the fault in the array is not repairable if the binary output of the BIST circuit indicates that the data read from the array do not correspond correctly to the test data written to the array for any of the different sets.
There is also provided, in accordance with a preferred embodiment of the present invention, a method for self repair of a memory array having a design size given by an integer n, the array including a matrix of memory cells arranged in n+1 lines, the method including:
selecting n of the n+1 lines to which data are to be written and from which the data are to be read;
testing the selected lines of the memory array by writing test data to the memory array, reading the data from the memory array, and comparing the data read from the array to the test data written thereto;
generating a binary output indicating whether or not the data read from the array correspond correctly to the test data written to the array; and
responsive to the binary output, repeating the above steps of selecting, testing and generating for different sets of n of the lines, so as to find one of the different sets for which the binary output indicates that the data read from the array correspond correctly to the test data written to the array.